A Humins-Derived Magnetic Biochar for Water Purification by Adsorption and Magnetic Separation

Abstract

In this study, the use of magnetic biochar particles recovered from biorefinery by-products (humins) for adsorption of hydrophilic organic pollutants was investigated. The biochar was prepared by thermal treatment of crude humins followed by a grinding step after which a magnetic iron oxide was co-precipitated on the biochar surface. The resulting iron oxide content of the biochar composite was found to be 9% by volume, and the presence of a characteristic Fe–O vibrational band was observed by FTIR-ATR. XPS analysis of Fe2p spectrum enabled the nature of iron oxide to be identified as maghemite. Finally, magnetometry measurements demonstrated the superparamagnetic properties of maghemite. The adsorption of methylene blue on the biochar composite was found to be fast (less than 1 h at pH 6 with an initial concentration of methylene blue of 2 × 10–5 mol L−1). Kinetics data were satisfactorily modelled by both first and second order models. Freundlich and Langmuir models were applied to adsorption isotherms data. Maximum adsorption capacity (3.35 × 10–5 mol g−1), and Langmuir and Freundlich constants (2.33 × 104 L mol−1 and 5.70 × 10–5 mol0.913 L0.087 g−1 respectively) were found to be comparable to the average of those found in the literature. Electrostatic attraction between oppositely charged methylene blue and magnetic biochar was presumed to be the dominant interaction governing adsorption at environmental pH values. Lastly, a laboratory-scale experimental device with magnetic filtration under flow allowed the complete separation of the magnetic biochar composite from the liquid phase. This study shows that this magnetic biochar composite is a promising and economically interesting recovery route for biorefinery by-products and could be used for adsorption purposes.